The reason that skirmish needs the xp boost is because not only does it have fewer ships in it, the ships that it does allow give lower attack values, thus less tactical attack, thus less xp overall. Even if skirmish had the same number of ships as a GB, you would still get less xp because of the lower level and smaller ships you're shooting at. So in order to balance against that deficit in attack value, the room needs to get a boost to bring it up to par. Until that happens the room will not be popular because even getting less attack in a BB1 in a GB, you would still get more xp for it at the end than getting twice that attack in a skirmish.
